Acoustic assessment of commercially important pelagic fish resources

 in the central and southern South China Sea in the summer 

of 2019 based on acoustic research 

Abstract: The South China Sea is one of the most diverse marine areas in the world. Since the 1990s, the fishery resources in the northern offshore areas of the South China Sea have shown a significant trend of decay, and a large amount of offshore fishing pressure has begun to shift to the central and southern South China Sea. The central and southern South China Sea is rich in fishery resources, including Thunnus albacares, Katsuwonus pelamis, Auxis rochei, Decapterus macrosoma and Selar crumenophthalmus, which have large potential for exploitation. Therefore, accurate assessment of fish stocks is of great significance for guiding fishery production and management in the South China Sea. In recent decades, fisheries acoustics has gradually emerged as a method for fisheries resource assessment. With the advantages of being fast, efficient, high sampling rate, short survey time, not destroying biological resources, and providing abundant spatial and temporal data, fisheries acoustics is suitable for the survey of pelagic biological resources and has been widely used in the assessment of pelagic fishery resources.

In order to assess the status of fishery resources in the central and southern South China Sea, scientific echosounder was used to investigate and evaluate the fishery resources in the central and southern South China Sea from June to July 2019 (summer). The investigation area was 109°00′117°30′E, 5°30′16°00′N and the acoustics survey area was about 2.45×105 nmile2 with 42 biological sampling stations by light falling net. The time to lure fish with light was 2 h in each station. After each haul, catches were randomly selected on the deck according to the category ratio, all catches were classified and counted, and then the body length (or fork length) and the weight of the catches were measured and recorded. The acoustic data were collected according to the preset survey route by the split beam Simrad EK80 portable scientific echosounder (120 kHz), and the analysis processing was mainly done with Echoview (version 7.1) software. 

The fishery resource density was estimated using the echo integration method. Before the acoustic data analysis, sea surface navigation bubbles, false bottom, plankton and machine signals were removed as interference noises. This survey was conducted by two fishing vessels cooperating with each other. Although the completion of the task was ensured to the maximum extent, it was inevitable that some routes would be lost. In order to fully explore the acoustic data and more intuitively and accurately assess the resource in the central and southern South China Sea, the acoustic data of every 20 nmiles were used as an acoustic assessment station in this study. Furthermore, acoustic integration value was assigned according to the catch ratio of the nearest biological sampling station. In order to exclude the influence of surface navigation noise, the starting water layer of data integration was set to be 10 m. The lower limit of data integration was set to be 200 m, taking into account both the habitat depth of the main catch and quality of the acoustic data. Due to the presence of plankton in the survey area, a suitable integration threshold was set to remove the weak scatters such as plankton. Through the test, the integration threshold was set at -80 dB, which could remove the interference of zooplankton to the greatest extent without losing the signal of the target species.

Results of survey indicated that, Thunnus albacares, Katsuwonus pelamis, Auxis rochei, Decapterus macrosoma and Selar crumenophthalmus were the dominant fish species in the survey, with occurrence rates of 71.43%，61.90%，30.95%，78.57% and 85.71%, respectively in 42 biological sampling stations. We measured the biological data of the main economic species: the length range of Thunnus albacares was 37276 mm, the average body length was 148.71 mm, the body mass range was 1.2469.6 g, and the average body mass was 93.15 g; the length range of Katsuwonus pelamis was 46479 mm, the average body length was 155.23 mm, the body mass range was 46479 g, and the average body mass was 97.08 g；the body length of Auxis roche ranged from 62314 mm, with a mean body length of 174.10 mm, and body mass ranged from 4.0657.6 g, with a mean body mass of 109.59 g；the body length of Decapterus macrosoma ranged from 20246 mm with a mean body length of 154.78 mm and body mass ranged from 4.0657.6 g with a mean body mass of 80.69 g；the body length of Selar crumenophthalmus ranged from 30206 mm, with a mean body length of 95.86 mm, and the body mass ranged from 0.8131.7 g, with a mean body mass of 19.31 g.

The biomass and abundance of five fish species were assessed by combining fishery acoustic data and biological data. Results of the acoustic assessment showed that the biomass of Thunnus albacares, Katsuwonus pelamis, Auxis rochei, Decapterus macrosoma and Selar crumenophthalmus was 2.81×104 t, 3.77×104 , 8.71×104 t, 5.73×104 t, 2.66×104 t and the abundance was 3.78×109 ind., 2.39×109 ind., 8.95×108 ind., 4.95×108 ind., 1.55×109 ind., respectively. Moreover, areas with the highest biomass density of the five fishes species were concentrated in 14°00′16°00′N, 109°00′117°30′E. Compared with historical survey data, the quantity and structure of fishery resources in the South China Sea have changed to some extent. The quantity of economic fish resources decreased to a certain extent, and showed the phenomenon of miniaturization and younger age. It’s worth noting that the biomass of tuna species, including Thunnus albacares, Katsuwonus pelamis, Auxis rochei has decreased significantly. Therefore, appropriate measures should be considered to protect the tuna resources in the South China Sea. In contrast, the biomass of Carangidae species, including Decapterus macrosoma and Selar crumenophthalmus, has not changed significantly, which means that Carangidae species can still be reasonably exploited. Our research can provide a scientific basis for rational development of fishery resources and development of reasonable conservation strategies in the offshore waters of the South China Sea.